Just had the same issue with mine. Turned out it was one of my alternators (generators) was getting ready to lock up and eventually did. From time of start to time that it actually locked up was about 2 weeks from the very first time to the demise. My gen light didn't come on because it was burned out. First indication I had was the squeeling stopped and the my lights started going dim, then the battery died when I tried to start the truck.
Other issue could also be the belt just not being tight enough.
If your alternator does go out, let me know. I have a rebuilt one that just came out of the shop last week.